A wide range of industrial applications require a bulk material to be separated or isolated into several constituent parts through, for example, some type of filtration process utilizing a liquid medium. Once filtered, the separated constituent, and/or the remainder of the bulk material and liquid medium, may be further processed so as to result in one or more desired products. By way of example, various methods of producing alcohol from grain may require that the fibrous constituent of the grain be separated from the starch and/or other constituents of the grain. A corn wet milling process, for example, separates the fiber from the starch in corn and subsequently uses the starch to produce ethanol, which may be used in automobiles or other motor vehicles. A dry grind milling process, for example, also separates its fiber, or insoluble solids (“wet cake”), from the liquid or “thin stillage” from the residuals, i.e., “whole stillage”, produced from distillation. Such fiber is subsequently used to produce Distillers Wet Grain with Soluble (DWGS) or Distillers Dried Grain with Soluble (DDGS). Filtration processes that separate a constituent from a liquid medium involve a step in other industrial applications as well. In this regard, the pulp and paper industry often requires the separation of fiber from a fibrous bulk material. Such filtration processes also exist in the textile manufacturing industry, the chemical industry (e.g., crystal formation applications), and other fields.
In a corn wet mill process, for example, to facilitate separation of the various constituents of the corn, the corn is mixed with water to form a slurry having a relatively high percentage of water (e.g., 80% or higher). The fiber is then filtered from the slurry, which contains, for example, starch and gluten constituents of the corn in addition to the water. In the corn wet mill process, conventional devices for filtering the fiber from the starch-containing slurry may include pressure screen devices and paddle screen devices. After initially filtering the fiber from the slurry, some of the starch and/or the gluten may still be associated with the fiber. Thus, it may be desirable to wash the fiber and remove additional amounts of starch and/or gluten therefrom. In this regard, the fiber is typically mixed with a liquid medium, such as wash water, and directed back through a pressure screen or a paddle screen device to separate the fiber from the wash water, which contains the additional starch and/or gluten washed from the fiber. The slurry is further processed to produce ethanol.
Conventional systems may include multiple washing stages to remove the starch and/or gluten from the fiber. For example, processing systems utilizing pressure or paddle screen devices typically include six or seven such stages. These various stages typically include separate, dedicated devices to facilitate washing of the fiber with wash water, which is then directed to a pressure screen or paddle screen device for filtration of the fiber therefrom. In addition, subsequent to washing, the fiber may be, or may need to be, de-watered, which can require yet another device. Alternatively, some devices are capable of both filtering and washing the fiber.
The pressure screen devices, for example, can direct the slurry to flow through a static screen under relatively low fluid pressure. The screen includes openings sufficiently sized so as to permit the water, starch, and gluten (any other constituents smaller than the openings) to flow through the screen, but prevent the fiber from flowing therethrough, thus essentially filtering the fiber from the slurry. Paddle screen devices include rotating paddles with a stationary drum including an outer wall configured as a screen. The rotation of the paddles directs the slurry toward the screened outer wall and essentially presses the slurry so as to force the water, the starch, and the gluten through the screen while preventing the fiber from passing therethrough. The movement of the paddles relative to the drum loosens the fiber from the outer wall and reduces plugging of the screen openings. Also, the centrifugal force created by the rotating paddles provides a higher filtration pressure as compared to the pressure screens. This higher pressure gives a higher capacity per unit screen surface, but larger sized particles can be forced through the screen in the paddle screen devices. These devices may include a washing stage, after which the fiber is filtered again. In many applications, pressure screen and/or paddle screen devices cannot dewater to the required dryness and another device, such as a screw press or drum filter under vacuum, is required to further dewater the solids.
Pressure screen devices may include a conical screen where the diameter varies. The diameter of these types of screen is commonly decreased as the material moves from a feed section to a discharge section. By decreasing the diameter of the cylindrical screen, the slurry is compacted. As the slurry becomes compacted, more of the water and smaller particles may be forced through the screen. However, this compression of the slurry may create a “mat” of solids, which can undesirably consume more energy. If the mat becomes too thick, the separation of the fiber from the water, the starch, and the gluten, for example, also may become more difficult. Because the energy required to process a given amount of bulk material increases as the diameter of the cylindrical screen increases, increasing the diameter of the cylindrical screens in paddle screen devices is generally undesirable.
Although such systems operate for their intended purpose, these systems have several drawbacks. For example, the washing of the fiber in these systems is typically inefficient leading to a relatively large number of stages. These large, multiple-stage systems, which often include multiple devices, represent significant capital and/or operating costs, as well as high maintenance costs for those devices. Furthermore, the above systems are prone to screen blinding and significant down time. For example, pressure screen systems generally require a high pressure washing about every eight hours of operation so as to function adequately.
Accordingly, there is a need for an improved apparatus and method for separating material, such as fiber, from a slurry or other liquid medium in a more efficient manner.